Cyclonic chambers are well known in the art and have been used in many applications, such as in separating, comminuting, mixing, and drying materials. In isolation, a cyclone is a simple mechanical device that can accomplish the above-listed tasks by using the force of gravity, centrifugal forces and pressure differentials at various points. Generally, cyclone chambers (hereinafter also referred to merely as "cyclones") are formed at least partially in the shape of an inverted cone, with the base (largest diameter) of the cone generally on top. Depending on their dimensions, the cyclones may also be in the shape of an inverted frustum, which is generally a cone shape where the small, tapered end has been cut off parallel to the base. Because cone-shaped cyclones and frustum-shaped cyclones are operationally similar, reference will be made herein primarily to a cone-shaped cyclone.
Cyclones may come in a variety of configurations that are intended for different applications. For example, as shown in FIG. 1, a cyclone is shown having a body 10 that comprises an upper, cylindrical shaped portion 12, and a lower, cone-shaped portion 11. FIG. 1 is described in the Handbook of Industrial Drying, pp. 728-733, at FIG. 11 (2nd Edition, Arun S. Mujumdar, editor, 1995). The cyclone shown and described there has three orifices for dust particles and air to enter and exit the cyclone. In the application described therein, an airstream containing dust particles enters the cyclone at an airstream input orifice 13, at a high velocity in a direction tangential to a center axis 14. The velocity is high enough so that the entering airstream is forced against the outside wall of the cyclone due to centrifugal forces. Gravity forces denser material (dust particles in this illustration) to fall, thereby resulting in a circular, downward vortex, as shown at 15. Gravity forces the dust particles eventually to escape through a bottom orifice 16 of the cyclone.
At the same time, a circular vortex is created that draws air upward inside the cyclone. This upward vortex 17 carries air and other particles up and out through an exit orifice 18. A number of factors determine which particles escape through the bottom orifice 16 or through the exit orifice 18. Among these factors are the pressures at each of the orifices, the velocity of the entering airstream and the velocity of each of the vortexes, the size and density of particles, the dimensions of the cyclone, and the interior structure of the cyclone. Generally, particles are carried upward via the upward vortex 17 when buoyant forces overcome the gravitational forces.
A cyclone such as that described above may be used to dry a wet substance as the substance is passed through the cyclone. Various methods have been used to effect the drying of the substance. For example, a wet substance may be introduced through the same tangential port where the high velocity airstream enters the cyclone. The substance is dried as the high velocity air impacts individual particles of the substance. Often, the air is heated to effect more efficient drying. Alternatively, the wet substance could be inserted separately at a point near where the tangential air stream enters the orifice, so that the air immediately impacts the substance and forces the substance to flow in a circular vortex. Another similar drying method uses a variant on the cyclone chamber, and is commonly called a spray dryer. A spray dryer operates by reducing the material to be dried into small droplets, then subjecting those small droplets to a large amount of hot air, thereby supplying the heat necessary to evaporate the liquid.
None of these prior dryers are able to efficiently dry large volumes of sticky, pasty material, such as paper pulp and municipal sludge. One of the problems with the prior dryers is that the sticky and pasty materials tend to stick to the sides of the cyclone. A second problem with prior dryers is their limited ability to suspend the wet material or otherwise keep it in the cyclone for a sustained period of time in order to most efficiently dry the material, resulting in an inadequate resonance time (alternatively known as retention time). Because of their structure, such prior dryers are limited in their operating parameters, such as the velocity of the incoming airstream and material feed, the volume of air allowed into the dryers and the pressure applied at the various inlets, particularly the airstream inlet. A third problem is that, in order to create a tangential flow of air in prior dryers, the fans and or ducting supplying the air need to be oriented in such a manner as to create a tangential flow. This can cause problems when installing the dryers, because the site may have to be modified to allow for specialized fans and ducting. Also, prior dryers are not as efficient as they could be.
Current environmental regulations and space constraints make it desirable to be able to dry sludge, paper pulp, and other wet materials, prior to removing them from their source. The efficiency of prior dryers, however, often is not high enough to make their use economical. The cost of drying and removing the material is generally compared to the cost of simply transporting and dumping the wet material.
The prior art dryers often are inefficient because the air inside the cyclone, even if it is heated and spinning rapidly, can only affect a small part of the surface area of the substance to be dried. Often, the air and the material to be dried create a smooth flowing mass, with little turbulence. While this creates a smooth airflow, it does not promote optimum drying. Their design limits the amount of pressure and inlet velocity that can be used efficiently. While it may be possible for spray dryers to be adapted to handle sticky and pasty material, their inefficiency and reliability is a drawback.